Comparison of the catalytic properties of untreated TiO2 nanotubes (left) and ones in which just the rims are coated with iron oxide (right) raises questions about key assumptions on these materials' photochemical properties.

Credit: J. Phys. Chem. C

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Comparison of the catalytic properties of untreated TiO2 nanotubes (left) and ones in which just the rims are coated with iron oxide (right) raises questions about key assumptions on these materials' photochemical properties.

Credit: J. Phys. Chem. C

Researchers in South Korea have obtained data that questions key assumptions about the photochemical properties of semiconductor nanotubes.

The work suggests that seemingly promising strategies for using nanotubes to boost the performance of solar cells and photocatalysts based on titanium dioxide, a well-studied candidate for those applications, may be less effective than expected. The study also identifies directions for follow-up investigations.

The study was presented at the American Chemical Society meeting in the Division of Fuel Chemistry at a symposium on solar energy conversion and has just been published in the Journal of Physical Chemistry C (DOI: 10.1021/jp201215t).

Compared with pure TiO2 nanoparticles, TiO2 nanotubes and nanotubes made from TiO2 hybridized with iron oxide are predicted to exhibit enhanced photo properties as a result of increased light scattering from the tubes' internal surfaces and broader light harvesting capabilities of the hybrid.

Tae Hwa Jeon and Hyunwoong Park of Kyungpook National University in Daegu, and coworkers, examined that prediction for the iron oxide-based nanotubes by preparing unhybridized TiO2 nanotubes and hybridized nanotubes with nanocrystalline hematite (α-Fe2O3) particles deposited on their rims or filling the nanotubes completely. The team used those materials as well as TiO2 nanoparticle films and other control samples to measure photocurrents generated in electrolyte solution under standard conditions. They used the same materials in a separate set of experiments to measure the rate of photocatalytic decomposition of phenol.

The key result reported by the group is that nanotubes containing hematite—especially rim-coated nanotubes, in which the interior tube surface remains exposed and available to mediate reactions—are far less active photochemically than uncoated samples.

"It's a surprising result that suggests that the interior surface of these nanotubes is not as photoactive as people would think," says Chad D. Vecitis, a Harvard University professor of environmental engineering. He adds that the work is likely "to bring up a new discussion and help push investigations in a direction that will provide answers."

Park proposed that the unexpectedly low activity could result from hematite-induced charge recombination, a process in which excitations induced by photons are quickly quenched by charge transfer at the hematite-nanotube interface. He added that a telling follow-up experiment, one that would home in on the photoactivity of the nanotubes' internal surfaces, would be to repeat the measurements—this time using nanotubes that have been selectively hybridized with photoinactive materials.